The present embodiment generally relates to Mach-Zehnder-configured interferometers and reference length standards in counter chirp FM laser radars. More specifically, methods and materials are described which alleviate interferometer drift due to vapor absorption observed for conventional fiber jacketing materials.
There are a number of optical systems that can measure a distance to a target. Such systems typically utilize an open beam propagated through free space between the laser source and the target. However, when the target location is such that limited free space is available for beam propagation, such systems are of limited use. Thus, well known systems may be able to perform distance measurements, but the open beam optical sensor head prevents application in limited access areas and tight places. For example, precision measurement of dimensions inside a chassis cannot easily be accomplished with such open beam systems. While it is known to transfer light through optical fibers, precision is compromised due to the environmental effects on the fiber itself. These environmental effects can change the optical path length and the polarization of the light in the fiber, adversely affecting measurement precision.
The existing art in precision FM laser radar incorporates a single chirp laser source and a polarization maintaining fiber optic geometry with separate local oscillator (LO) and signal paths (see, e.g., U.S. Pat. Nos. 4,824,251 and 4,830,486). Such laser radars most typically use a length standard of some kind as a basis for a high-precision absolute length measurement.
The existing art in length standards for these and related applications fall into two primary categories. One category involves an artifact based standard such as temperature compensated metal or glass bars/tubes with targets mounted to each end. For the range needed (2-4 m) these are unwieldy and it is impractical to integrate them into a measurement system. The second category utilizes a fiber optic based length standard, which can easily be packaged into a small volume.
In an improvement on the single chirp laser system referred to above, the inventor herein has developed a counter chirp configuration that provides a much greater insensitivity to vibration induced range errors by providing for a more accurate Doppler correction. Moreover, by combining the LO and signal paths for two lasers into a single fiber, the fiber optic circuit is both less complicated and less expensive due to fewer components and completely immune to error caused by changes in the LO and signal path lengths due to environmental factors such as temperature. This configuration is described in U.S. patent application Ser. No. 11/354,382, filed Feb. 15, 2006, the contents of which are incorporated herein by reference as though set forth in their entirety. That technology utilized reference standards either of the Mach-Zehnder type or of the Michelson type. However, that application did not disclose methods of preventing non-temperature-related drift.
Ahmadvand et al. (U.S. Pat. No. 6,778,278) disclose a method to compensate for temperature drift in a Mach-Zehnder interferometer by adding jacketing material of a specific thickness and length to one or both arms of the interferometer as a means of eliminating the need for active temperature control of the device. The result is that the two arms change the same amount with temperature changes despite the fact that they are different lengths. However, for higher channel density applications differences in path length will increase (as would the need for the temperature compensation). Moreover, they do not address other environmental causes of interferometer drift.
Bauer et al. (U.S. Pat. No. 6,757,469) disclose fabrication methods of waveguide devices that insure temperature insensitivity. These waveguide devices are not optical fibers. Again, the disclosure is limited to temperature effects not alleviation of vapor absorption effects.
Typically, the Mach-Zehnder interferometer formed by two couplers and the fiber between them is kept in a temperature-controlled container to prevent the fiber lengths from changing. If the difference in fiber lengths is calibrated, the reference interferometer can serve as an absolute length standard for the laser radar system as well as provide a signal useful in the linearization of the laser waveform.
FM lasers are largely immune to ambient light conditions and changes in surface reflectivity because FM laser radars rely only on beat frequency, which is not dependent upon signal amplitude, to calculate range. This enables the FM Coherent system to make reliable measurements with as little as one picowatt of returned laser energy. This corresponds to a nine order-of-magnitude dynamic range of sensitivity. However, these instruments are not immune to drift caused by other ambient conditions.
Ambient conditions other than temperature can affect effective optical path lengths in several ways. In particular, the presence of water vapor in the surrounding air affects the optics. In addition to water vapor, volatile components of the polymers (especially acrylate) can outgas, thereby causing a dimensional change in the jacket and thus a change in the fiber as described below.
An effective optical path length in an optical fiber can change either because of a change in the refractive index of the core of the fiber, potentially caused by absorption of impurities, or because of a physical change in diameter or length of the fiber brought about by ambient conditions. If a fiber's jacket absorbs, for example, water vapor or other impurities, the jackets dimensions change, thus exerting compressive force on the underlying glass fiber. Conversely, outgassing of water vapor or other impurities diminishes compressive force on the glass fiber. The change in force causes both the diameter and length of the fiber to change by small amounts and possibly the index of refraction to change, causing a change in effective optical path length. Since the precision being sought in the current embodiment is on the order of one part per million, it can be seen that tiny changes in dimension or optical path length can affect the precision of the instrument. The change in force can also degrade the polarization maintainability of the fiber due to stress birefringence.
What is needed is a jacketing/coating material that does not tend to absorb or outgas water vapor and other impurities which can cause an effective change in the length of the reference arm.